The mucopolysaccharidoses are a group of inherited disorders caused by a lack of specific lysosomal enzymes involved in the degradation of glycosaminoglycans (GAG), also called mucopolysaccharides. The accumulation of partially-degraded GAG causes interference with cell, tissue, and organ function. Over time, the GAG accumulates within cells, blood, and connective tissue, resulting in increasing cellular and organ damage. The most serious of the mucopolysaccharidosis (MPS) disorders, MPS I, is caused by a deficiency of the enzyme α-L-iduronidase (IDUA). This leads to three clinical syndromes which in order of severity are Hurler, Hurler-Scheie and Scheie syndromes. Each is inherited in an autosomal recessive manner with the extent of enzyme deficiency being directly related to the severity of the clinical phenotype.
The IDUA gene has been reported to provide instructions for producing an enzyme called alpha-L-iduronidase, which is essential for the breakdown of large sugar molecules called glycosaminoglycans (GAGs). Specifically, alpha-L-iduronidase is reported to remove sulfate from a molecule known as sulfated alpha-L-iduronic acid, which is present in two GAGs called heparan sulfate and dermatan sulfate. Alpha-L-iduronidase is located in lysosomes, compartments within cells that digest and recycle different types of molecules. More than 100 mutations in the IDUA gene have been found to cause mucopolysaccharidosis type I (MPS I). Mutations that change one DNA building block (nucleotide) are the most common. Mutations that cause MPS I to reduce or completely eliminate the function of alpha-L-iduronidase.
With respect to the clinical syndromes, the current standard of care for Hurler syndrome is hematopoietic stem cell transplantation (HSCT) such as bone marrow transplantation (BMT) or umbilical cord blood transplantations (UCBT). The procedure is done as early as possible, and before the age of two, to impact on both somatic and CNS aspects of the disease. However, HSCT for MPS I remains associated with a significant amount of morbidity and a 20% mortality rate. If transplantation is not an option, then enzyme replacement therapy (ERT) may be started which requires a weekly infusion of enzyme for the life of the patient. ERT does not impact on the progression of CNS disease but does partially improve the somatic manifestations. Organomegaly is significantly improved although aspects of the disease in the skeletal system, eye and heart are only partially improved. Patients may require surgery to stabilize the hip and knee and to treat carpal tunnel syndrome and finger contractions. Cardiac disease is treated medically although surgery may eventually be required.
ERT for MPS I provides exogenous enzyme for uptake into lysosomes and increased catabolism of GAG. Although the lysosomal enzymes function internally, cell-surface mannose-6-phosphate receptors are capable of binding, internalizing, and delivering these enzymes to the lysosomes. Recombinant IDUA (Aldurazyme®, BioMarin) is approved by FDA for patients with Hurler and Hurler-Scheie forms of MPS I and for patients with the Scheie form who have moderate to severe symptoms and was shown to improve pulmonary function and walking capacity. ERT has also been observed to reduce hepatomegaly in MPS I patients, as well as the levels of urinary GAG. However, because intravenous enzyme does not easily cross into the brain, ERT does not currently address the neurological symptoms experienced by some MPS I patients.
Complications of ERT revolve around immune response to the recombinant enzyme which can range from mild to full-blown anaphylaxis as well as complications of life-long peripheral access such as local and systemic infections. Up to 91% of patients receiving Aldurazyme develop antibodies to the enzyme, although it is not clear how much it affects efficacy. Furthermore, ERT requires weekly intravenous (i.v.) infusions, administered over a period of 3-8 hours in a hospital setting, which significantly impacts patient quality of life and, at a high expense, is a major strain on health care reimbursement systems.
In light of these limitations, a treatment that can more effectively correct the morbidity associated with MPS I remains an unmet medical need.